From review research from Australia's The University of Western Australia we find a study that gave 146 people either a garlic supplement - standardized to 180 milligrams of allicin - or a placebo for 12 weeks. The research found that the placebo group in total had 65 common cold occurrences while the garlic group only had 24 occurrences. This is less than half the number of colds.

Furthermore, when those who were taking the garlic supplement did catch a cold, that cold lasted an average of one day shorter than the colds among the placebo group – some 20-25% shorter.

Another study, from the University of Florida, tested 120 people by giving half 2.5 grams per day of an aged garlic extract supplement and the other half a placebo. Over a six month period, the garlic group had 61 percent fewer number of days of colds, and 58 percent had few incidences of colds, along with 21 percent fewer cold symptoms when they did catch a cold.

The Common Cold With a Capital C – Vitamin C

With regard to vitamin C, a Cochrane review of 29 clinical trials that including 9,676 cold episodes found a "consistent" benefit of 8 percent reduction of cold duration among adults and 14 percent reduction of cold duration among children. The large metadata calculation utilized vitamin C doses over 200 milligrams per day.

With regard to higher doses, better results were found. Children given between 1,000 and 2,000 milligrams of vitamin C a day had an average of 18 percent shorter colds.

This review also found that regular vitamin C supplementation reduced the severity of colds.

What About Zinc and the Common Cold?

Another Cochrane database review performed last year analyzed 16 therapeutic studies that included 1,387 people, plus two prevention trials. The research found that zinc taken within 24 hours of the appearance of cold symptoms significantly reduced the duration of a cold.

There was a 55 percent reduction in the number of people symptomatic of a cold after seven days compared to those not taking zinc. And the incidence of colds among those taking zinc regularly was 36 percent less than those who were not.